1 files changed, 264 insertions, 0 deletions

diff --git a/crates/ra_assists/src/handlers/merge_match_arms.rs b/crates/ra_assists/src/handlers/merge_match_arms.rs
new file mode 100644
index 000000000..670614dd8
--- /dev/null
+++ b/crates/ra_assists/src/handlers/merge_match_arms.rs
@@ -0,0 +1,264 @@
+use std::iter::successors;
+
+use ra_syntax::{
+    ast::{self, AstNode},
+    Direction, TextUnit,
+};
+
+use crate::{Assist, AssistCtx, AssistId, TextRange};
+
+// Assist: merge_match_arms
+//
+// Merges identical match arms.
+//
+// ```
+// enum Action { Move { distance: u32 }, Stop }
+//
+// fn handle(action: Action) {
+//     match action {
+//         <|>Action::Move(..) => foo(),
+//         Action::Stop => foo(),
+//     }
+// }
+// ```
+// ->
+// ```
+// enum Action { Move { distance: u32 }, Stop }
+//
+// fn handle(action: Action) {
+//     match action {
+//         Action::Move(..) | Action::Stop => foo(),
+//     }
+// }
+// ```
+pub(crate) fn merge_match_arms(ctx: AssistCtx) -> Option<Assist> {
+    let current_arm = ctx.find_node_at_offset::<ast::MatchArm>()?;
+    // Don't try to handle arms with guards for now - can add support for this later
+    if current_arm.guard().is_some() {
+        return None;
+    }
+    let current_expr = current_arm.expr()?;
+    let current_text_range = current_arm.syntax().text_range();
+
+    enum CursorPos {
+        InExpr(TextUnit),
+        InPat(TextUnit),
+    }
+    let cursor_pos = ctx.frange.range.start();
+    let cursor_pos = if current_expr.syntax().text_range().contains(cursor_pos) {
+        CursorPos::InExpr(current_text_range.end() - cursor_pos)
+    } else {
+        CursorPos::InPat(cursor_pos)
+    };
+
+    // We check if the following match arms match this one. We could, but don't,
+    // compare to the previous match arm as well.
+    let arms_to_merge = successors(Some(current_arm), next_arm)
+        .take_while(|arm| {
+            if arm.guard().is_some() {
+                return false;
+            }
+            match arm.expr() {
+                Some(expr) => expr.syntax().text() == current_expr.syntax().text(),
+                None => false,
+            }
+        })
+        .collect::<Vec<_>>();
+
+    if arms_to_merge.len() <= 1 {
+        return None;
+    }
+
+    ctx.add_assist(AssistId("merge_match_arms"), "Merge match arms", |edit| {
+        let pats = if arms_to_merge.iter().any(contains_placeholder) {
+            "_".into()
+        } else {
+            arms_to_merge
+                .iter()
+                .flat_map(ast::MatchArm::pats)
+                .map(|x| x.syntax().to_string())
+                .collect::<Vec<String>>()
+                .join(" | ")
+        };
+
+        let arm = format!("{} => {}", pats, current_expr.syntax().text());
+
+        let start = arms_to_merge.first().unwrap().syntax().text_range().start();
+        let end = arms_to_merge.last().unwrap().syntax().text_range().end();
+
+        edit.target(current_text_range);
+        edit.set_cursor(match cursor_pos {
+            CursorPos::InExpr(back_offset) => start + TextUnit::from_usize(arm.len()) - back_offset,
+            CursorPos::InPat(offset) => offset,
+        });
+        edit.replace(TextRange::from_to(start, end), arm);
+    })
+}
+
+fn contains_placeholder(a: &ast::MatchArm) -> bool {
+    a.pats().any(|x| match x {
+        ra_syntax::ast::Pat::PlaceholderPat(..) => true,
+        _ => false,
+    })
+}
+
+fn next_arm(arm: &ast::MatchArm) -> Option<ast::MatchArm> {
+    arm.syntax().siblings(Direction::Next).skip(1).find_map(ast::MatchArm::cast)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::merge_match_arms;
+    use crate::helpers::{check_assist, check_assist_not_applicable};
+
+    #[test]
+    fn merge_match_arms_single_patterns() {
+        check_assist(
+            merge_match_arms,
+            r#"
+            #[derive(Debug)]
+            enum X { A, B, C }
+
+            fn main() {
+                let x = X::A;
+                let y = match x {
+                    X::A => { 1i32<|> }
+                    X::B => { 1i32 }
+                    X::C => { 2i32 }
+                }
+            }
+            "#,
+            r#"
+            #[derive(Debug)]
+            enum X { A, B, C }
+
+            fn main() {
+                let x = X::A;
+                let y = match x {
+                    X::A | X::B => { 1i32<|> }
+                    X::C => { 2i32 }
+                }
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn merge_match_arms_multiple_patterns() {
+        check_assist(
+            merge_match_arms,
+            r#"
+            #[derive(Debug)]
+            enum X { A, B, C, D, E }
+
+            fn main() {
+                let x = X::A;
+                let y = match x {
+                    X::A | X::B => {<|> 1i32 },
+                    X::C | X::D => { 1i32 },
+                    X::E => { 2i32 },
+                }
+            }
+            "#,
+            r#"
+            #[derive(Debug)]
+            enum X { A, B, C, D, E }
+
+            fn main() {
+                let x = X::A;
+                let y = match x {
+                    X::A | X::B | X::C | X::D => {<|> 1i32 },
+                    X::E => { 2i32 },
+                }
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn merge_match_arms_placeholder_pattern() {
+        check_assist(
+            merge_match_arms,
+            r#"
+            #[derive(Debug)]
+            enum X { A, B, C, D, E }
+
+            fn main() {
+                let x = X::A;
+                let y = match x {
+                    X::A => { 1i32 },
+                    X::B => { 2i<|>32 },
+                    _ => { 2i32 }
+                }
+            }
+            "#,
+            r#"
+            #[derive(Debug)]
+            enum X { A, B, C, D, E }
+
+            fn main() {
+                let x = X::A;
+                let y = match x {
+                    X::A => { 1i32 },
+                    _ => { 2i<|>32 }
+                }
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn merges_all_subsequent_arms() {
+        check_assist(
+            merge_match_arms,
+            r#"
+            enum X { A, B, C, D, E }
+
+            fn main() {
+                match X::A {
+                    X::A<|> => 92,
+                    X::B => 92,
+                    X::C => 92,
+                    X::D => 62,
+                    _ => panic!(),
+                }
+            }
+            "#,
+            r#"
+            enum X { A, B, C, D, E }
+
+            fn main() {
+                match X::A {
+                    X::A<|> | X::B | X::C => 92,
+                    X::D => 62,
+                    _ => panic!(),
+                }
+            }
+            "#,
+        )
+    }
+
+    #[test]
+    fn merge_match_arms_rejects_guards() {
+        check_assist_not_applicable(
+            merge_match_arms,
+            r#"
+            #[derive(Debug)]
+            enum X {
+                A(i32),
+                B,
+                C
+            }
+
+            fn main() {
+                let x = X::A;
+                let y = match x {
+                    X::A(a) if a > 5 => { <|>1i32 },
+                    X::B => { 1i32 },
+                    X::C => { 2i32 }
+                }
+            }
+            "#,
+        );
+    }
+}